1. Field of the Invention
The present invention relates to spiral fabrics. More specifically, the present invention relates to spiral-link fabrics having coils with relatively low monofilament diameter to width ratio utilized on a papermaking machine and other industrial applications.
2 . Description of the Related Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in a forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Fabrics in modern papermaking machines may have a width of from 5 to over 33 feet, a length of from 40 to over 400 feet and weigh from approximately 100 to over 3,000 pounds. These fabrics wear out and require replacement. Replacement of fabrics often involves taking the machine out of service, removing the worn fabric, setting up to install a fabric and installing the new fabric.
For example, because of the solid support beams for dryer sections, all dryer fabric must have a seam. Installation of the fabric includes pulling the fabric body onto a machine and joining the fabric ends to form an endless belt. The seam region of any workable fabric must behave in use as close to the body of the fabric in order to prevent the periodic marking by the seam region of the paper product being manufactured.
A fabric may be formed completely of spiral coils (so called “spiral-link fabric”) as taught by Gauthier, U.S. Pat. No. 4,567,077; which is incorporated herein by reference. In such a fabric, cross-machine direction (“CD”) rows of spiral coils are connected to each other by at least one connecting pin, pintle or the like. The coils are wound or formed on a mandrel of a monofilament having a specific diameter 60. The coils, as shown for example in FIG. 4, have a major dimension (coil width) 50 and a minor dimension (coil thickness) 100. The coil width as referred to herein is actually the length of the coil as measured in the machine direction (“MD”) of the spiral-link fabric in use. Once the coil is in fabric form, the calculated fabric thickness or caliper, therefore, would be equal to twice the diameter of the monofilament that form the spiral coils plus the pintle diameter.
Actual measured values may, however, be higher dependent on the processing and finishing options used.
In theory, the seam can be at any location in the fabric body where a connecting pin may be removed. Spiral-link fabrics offer a number of advantages over traditional fabric. For example, the seam of a spiral-link fabric is geometrically similar to the fabric body, and thus is less likely to mark the paper sheet. In addition, spiral-link fabrics may withstand flattening, thus imparting constant permeability to fluids (in particular air) which would otherwise pass therethrough. Due to these advantageous features, spiral-link fabrics are used in papermaking machines, particularly for drying sheets of paper wherein water vapor is removed which passes through the spiral-link fabric. Spiral-link fabrics have other industrial applications where they act as industrial conveyors and may be coated or otherwise impregnated with a resin depending upon the application.
In current practice, the above characteristics of the coils, i.e. the diameter of the monofilament forming the spiral fabric and the resultant width to thickness ratio, are dependently related. Bigger coils with relatively large width and higher thickness must be formed from a thicker monofilament, whereas monofilaments of a relatively small diameter can only form smaller coils. The use of bigger coils made from thicker monofilaments and heavier pintles result in heavier fabrics i.e. more weight per unit area, higher thickness and caliper, with a coarse hand and low number of contact points. In order to make a fabric more suitable for the dryer section and other “mark-sensitive” positions of the papermaking machine, a finer, lighter weight and thinner fabric with higher number of contact points and smaller contact points is desired. However, these fabrics require the smaller coils made from smaller monofilaments in order to achieve the required characteristics. The disadvantage being that more rows of coils, pintles and stuffers will be needed to make an equivalent fabric of the same weight per unit area.
Additionally, the production of spiral-link fabrics is labor-intensive. For example, spiral-link fabrics are constructed of many small spiral elements that must be coiled and assembled. The multiple manufacturing steps of coiling, interdigitating, and interconnecting spiral coils is labor intensive and requires come specialized machinery. In addition, it is difficult to interconnect the spiral coils because a pin, pintle or the like is inserted through small channels formed from the interdigitated spiral coils. Production time for such fabrics is compounded because the small width of the spiral coils requires a large number of pintles, as fabrics may be formed in a width of from 5 to over 33 feet in CD and a length of from 40 to over 400 feet. Further, the large number of pintles substantially covers the fabric resulting in a fabric that is stiff both in CD and diagonally across the MD during operation.
In addition, “stuffers” in the form of yams or the like are typically inserted within the inner space of each spiral coil to usually lower the permeability of the fabric. Currently, stuffers are pushed or stuffed into the inner space of each spiral coil one portion at a time using specialized machinery. As is to be appreciated, such stuffing method limits the material which may be used as stuffers because the stuffer must be sufficiently stiff or rigid to facilitate insertion into the small coil opening and across the full width of the fabric. Further, because the stuffers are pushed into the fabric, the process of inserting the stuffers may be slow and labor-intensive.
The present invention overcomes these shortcomings by providing a spiral-link fabric with wide spiral coils.